As the number of glass fibers produced by a single glass fiber producing bushing has increased, the size of the bushing has increased. It has been necessary to compartmentalize some of these large bushings into segments as disclosed in Grubka, U.S. Pat. No. 4,272,272 which is hereby incorporated by reference. It has been difficult to measure the temperature of these large bushings across the face of the bushing. Tretheway, U.S. Pat. No. 3,246,124, used multiple thermocouples to sense the temperature at various points on the bushing and then averaged these readings to effectuate process control. Johnson, U.S. Pat. No. 3,820,967, removed the thermocouples from the bushing and placed them below the bushing to maintain control. Jensen, U.S. Pat. No. 4,024,336, used two thermocouples to sense the temperature at two points on the bushing to vary the power supplied to each segment of the bushing. Thermocouples on the bushing are prone to premature failure due to the high heat generated by the bushing. There is also a finite time lag between a temperature change on the bushing and a change in the reading produced by a thermocouple. The thermocouple outputs a millivoltage reading while the bushing is carrying thousands of amperes of alternating current which can easily mask the thermocouple signal.
Non-contacting measurements, such as infrared, have also been tried to measure the temperature of the bushings. Wakasa, U.S. Pat. No. 4,130,406, sensed the breakage of fibers with an infrared detector. Shofner, U.S. Pat. No. 4,343,637, used a similar detection system to control the process including supplying power to the bushing. Direct infrared temperature measurement of the bushing has always been imprecise due to the presence of the issuing streams of molten glass and the crowded conditions under the bushing caused by devices such as fin shields as disclosed in Stream, U.S. Pat. No. 4,153,438.
The present application overcomes the problems associated with both thermocouple and infrared temperature measurements by determining the temperature of each segment of the bushing by calculating the resistance of the bushing. A physical relationship, known as the temperature coefficient of resistance, exists between the resistance of a metal and the temperature of the metal. The resistance of a segment of the bushing can be determined by measuring the voltage drop across the segment when a known current is passed through the bushing. By using the temperature coefficient of resistance, a change in resistance of a segment of the bushing can be used to determine a temperature change.